Summary- Substantial progress was made on this project, with six original research articles and two reviews published. Along with our collaborators, we have characterized the pharmacological effects of numerous synthetic cannabinoids, cathinones, and opioids found in the street drug marketplace. In a representative study, we described the pharmacodynamics and pharmacokinetics of carfentanil, an ultrapotent fentanyl analog that has been implicated in hundreds of overdose deaths. Importantly, we show that carfentanil displays non-linear kinetics after systemic administration in rats, suggesting impaired clearance for this toxic substance. The current opioid overdose crisis is being exacerbated by illicitly manufactured fentanyl and its analogs. Carfentanil is a fentanyl analog that is 10,000-times more potent than morphine, but limited information is available about its pharmacology. Our study had two aims: 1) to validate a method for quantifying carfentanil and its metabolite norcarfentanil in small-volume samples, and 2) to use the method for examining pharmacodynamic-pharmacokinetic relationships in rats. The analytical method involved liquid-liquid extraction of plasma samples followed by quantitation of carfentanil and norcarfentanil using ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). The method was validated following SWGTOX guidelines, and both analytes displayed limits of detection and quantification at 7.5 and 15 pg/mL, respectively. Male Sprague-Dawley rats fitted with jugular catheters and temperature transponders received subcutaneous carfentanil (1, 3 and 10 ug/kg) or saline. Repeated blood specimens were obtained over 8h, along with pharmacodynamic measures including core temperature and catalepsy scores. Carfentanil produced dose-related hypothermia and catalepsy that lasted up to 8h. Carfentanil Cmax occurred at 15min whereas metabolite Cmax was at 1-2h. Concentrations of both analytes increased in a dose-related fashion, but area-under-the-curve values were much greater than predicted after 10 ug/kg. Plasma half-life for carfentanil increased at higher doses. Our findings reveal that carfentanil produces marked hypothermia and catalepsy, which is accompanied by nonlinear accumulation of the drug at high doses. We hypothesize that impaired clearance of carfentanil in humans could contribute to life-threatening effects and lead to re-narcotization after initial naloxone rescue.